In the etching of printed circuit boards (PCBs), an etchant is continuously sprayed onto pre-developed copper-clad laminates, etching away the unprotected copper to form circuits. It has been a common practice to use either acidic cupric chloride or ferric chloride etching method for copper etching. The acidic cupric chloride etching method relies on the use of an acidic cupric chloride etchant, the main components of which are: (1) cupric chloride, the main copper-etching agent, (2) an oxidant and (3) a sub-etchant that mainly contains hydrochloric acid. The composition of the etchant changes continuously as it reacts with copper-clad laminates during etching, therefore an automatic detection and charging control machine is widely employed to detect various parameters of the etchant, such as the oxidation-reduction potential (ORP), specific density, concentration of dissociated hydrogen ions, etc. Upon detection of these parameters, the machine calculates and charges appropriate amounts of oxidant, water and sub-etchant to the etchant to automatically and continuously regenerate the etchant.
In the above-mentioned etching process, the continuous charging of oxidant, water and sub-etchant inevitably results in an overflow of excess etchant from the etching tank, forming the so-called “etchant waste”. The main components of such etchant waste of an acidic cupric chloride etchant are: cupric chloride, cuprous chloride, hydrochloric acid and chloride salts, with concentration of copper ions typically higher than 50 g/L (0.8 mol/L). In industry, the etchant waste is usually collected and treated by hazardous chemical waste recycling companies, where they are converted into cupric sulphate or tribasic copper chloride. Some PCB manufacturing factories choose to self-treat the etchant waste by electrolysis. During electrolysis, the following reduction reactions take place on the cathode board:Cu2++2e−→Cu++e−→Cu;  (1)2H++2e−→H2.  (2)
On the anode board, the following oxidation reaction simultaneously takes place:2Cl−→Cl2+2e−.
While the copper metal formed on cathode board can be removed and reused, poisonous chlorine gas is liberated from the anode board. It has been observed that part of the Cu(I) ions in the electrolyte are oxidized to Cu(II) ions by some of the chlorine gas formed. By adding hydrochloric acid and other chemicals to this oxidized electrolyte, the sub-etchant can be regenerated, which can be then added to the etching production line to take part in etching.
However, the etching and electrolysis methods discussed above have the following disadvantages:
1. The chlorine gas generated is not completely absorbed by the electrolyte: as discussed above, the Cu(I) ions in conventional acidic cupric chloride etchant wastes react with gaseous chlorine in electrolysis. It has been observed that when in excess, Cu(I) ions tend to encourage a formation of copper sludge precipitate during etching. This precipitate greatly slows down etching rate to a point that industrial production requirements are not met. Therefore, the concentration of Cu(I) ions in the etchant needs to be kept relatively low. Consequentially, Cu(I) ion concentration in the etchant waste is low, and most of the chlorine gas from electrolysis does not have sufficient time to react with Cu(I) ions before escaping to the atmosphere, i.e. the electrolyte is unable to fully absorb and consume the large amount of poisonous chlorine gas produced in electrolysis. Chlorine gas treatment is therefore necessary. At present, the two chlorine treating methods described below are widely used:(A) Sodium hydroxide spray treatment: chlorine gas is vacuumed from the electrolysis tank to a neutralising spray absorption tower by an exhaust system. It is then neutralised by a sodium hydroxide spray. This method requires a large amount of sodium hydroxide for chlorine treatment, and the chlorine gas cannot be reused.(B) Etchant waste spray treatment: similar to sodium hydroxide spray treatment, chlorine gas is vacuumed from the electrolysis tank to a spray absorption tower, before being treated by an etchant waste spray. The chlorine gas oxidizes the etchant waste in this process, and the oxidized etchant waste can be collected and reused in etching. In this method, although some chlorine gas can be reused, there still remains a substantial amount of unreacted chlorine gas and HCl gas due to the high concentration of volatile HCl and low concentration of Cu(I) ions in the etchant waste. The chlorine gas and HCl gas need to be treated again in another neutralising spray absorption tower. When both installed indoors, the two spray absorption towers undesirably take up much workspace. If one or both spray absorption towers are installed outdoors, long tubes are required to transmit the chlorine gas stream, which leads to increased possibility of chlorine leakage, posing severe safety risks.2. Uncontrollable hydrogen gas formation: acidic cupric chloride etchant wastes tend to contain a large amount of hydrogen ions. In the case of high current density or exceedingly low copper ion concentration, a substantial amount of hydrogen gas tends to form at the cathode, which may lead to explosion.3. Low electrolysis current efficiency: current efficiency is the ratio of the actual mass of a substance liberated from an electrolyte by the passage of current to the theoretical mass liberated according to Faraday's law. During the above-mentioned electrolysis process, Cu(II) ions are firstly reduced to Cu(I) ions, which are subsequently reduced to metal copper. A low current efficiency is typically observed, possibly due to (a) some Cu(II) ions not having enough electrical energy to fully convert to metal copper, and (b) the intermediate Cu (I) ions attacking and corroding the metal copper formed.4. Low level of synchronisation: in current industry practice, etching, electrolysis, regeneration and recycling processes are typically separated. There lacks a unified system to allow process synchronisation. It is difficult, for example, to flexibly adjust the etching production parameters or the composition of the etchant in order to match the rate of electrolysis and the subsequent etchant regeneration and recycle process.5. Large amounts of raw materials required: in electrolysis, due to substantial hydrogen gas formation, the concentration of hydrogen ions in the electrolyte is usually very low. In order to regenerate the sub-etchant from the electrolyte, a large amount of hydrochloric acid is required. In etching, liquid oxidants are continuously added to oxidize Cu(I) ions in the etchant. The addition of oxidants, hydrochloric acid and other raw materials results in increased cost of not only production, but also storage and further treatment due to the continual increase of etchant volume.
Electrolytic recycling and regeneration methods specially designed for acidic cupric chloride etchant wastes have been proposed. In these methods, anode boards with rare earth coatings are employed, in an attempt to decrease chlorine and hydrogen gas evolution. However, none of these methods have been in wide spread use, probably due to the high fabrication and maintenance cost for such anode boards.
Chinese patent 201510117884.3 offers an improved acidic cupric chloride etchant. Ferric chloride is added to the etchant composition as an auxiliary etching agent. When operating in a high-acidity system, the said etchant increases etching rate while maintaining good etching quality. When operating in a low-acidity system, etching quality is improved, and etching rates are maintained at a level that is comparable to conventional acidic cupric chloride etchants. The said etchant comprises cupric chloride, a sub-etchant and an oxidant. The sub-etchant comprises the following components in weight percentage: 1%-36.5% of HCl, 0.01%-45% of one or more compounds selected from FeCl3, FeCl2, Fe, FeO and Fe2O3 and the balance of water. The control parameters of a production process using the said etchant are set as follows: the concentration of dissociated hydrogen ions is 0.1-5.0M, the ORP is 380-700 mV and the concentration of copper ions is 1-180 g/L. The present invention is a method designed for the recycling and regeneration of the said acidic cupric chloride etchant.